On 18/04/2016 10:21 am, John Clark wrote:
On Sun, Apr 17, 2016 Bruce Kellett <[email protected]
<mailto:[email protected]>>wrote:
>
Consider the following: Alice and Bob perform their experiments on
the entangled pair and record the results (magnet orientation and
outcome) in their lab books. They then go on with other things.
Some weeks later they meet up at a cafe down the street for a
coffee, and there compare their results. This is the first time
that any explicit information about magnet orientations and
results has ever been exchanged. And yet Alice and Bob meet in the
same world
Alice and Bob meet
in 4 different worlds because there are 4 different ways their lab
books could be.
There is only one Alice and one Bob in the one world I am talking about.
Other worlds can do their own thing -- they are totally independent.
>
How do you prevent the situation in which, when they compare their
lab books, they find that they both used the same magnet
orientation and both recorded |+>?
If the a massive particle decays into 2 lighter particles such that
one is spin up and the other is spin down then the world splits into
2, and in one I get the spin up and you get the spin down and in the
other I get spin down and you get spin up.
That is not the EPR case.
In no universe do we both get spin up because that would violate the
laws of physics. The MWI doesn't say everything happens, it just says
everything that doesn't contradicts the laws of physics happens.
Unfortunately, in order to avoid violations of quantum mechanics in this
situation, you need non-locality.
>
The situation before even more bizarrely extreme if you consider
the situation in which Bob and Alice do a long series of trials of
the experiment before they meet to compare notes. They each have a
series of |+> and |-> results, with corresponding orientations,
But the probabilities calculated from these sequences must match
the quantum predictions, even though no intermediate exchange of
orientation information ever took place. Note also that if the
initial separation is sufficient, or if the repeat rate was
sufficiently high, they could both accumulate these long sequences
of results before their future light cones ever intersected. The
records are fixed and unchangeable before any information exchange
ever takes place.
We know from experiment that things are bizarre. Suppose that Bob is
10 light years from Alice but they are not moving with respect to one
another, then there are in the same frame of reference and can agree
on simultaneity. So simultaneously they both intercept a stream of
entangled particles that originated a billion years ago a billion
light yeas away and they measure the spins of their particles, and
they both get a apparently random string of ups and downs and they
record the results on a paper. Alice then gets into her spaceship,
which moves at 99% of the speed of light and after 10 years meets Bob
and they compare records. Only then do they realize that Bob's
apparently random sequence of up's and downs are exactly the same as
Alice's.
Thus either the entanglement propagate
s instantly and things are not local, or the paper records change
depending on who is looking at them and things are not realistic, or
things are not deterministic and the identical sequence of
measurements was just a astronomically unlikely coincidence.
You appear to have a clear grasp of the situation -- either QM is
intrinsically non-local, or we have magic......
Bruce
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